Selective preparation of tetrahydrofuran by hydrogenation of maleic anhydride

ABSTRACT

A catalyst for the hydrogenation of C 4 -dicarboxylic acids and/or derivatives thereof, preferably maleic anhydride, in the gas phase comprises
         a) 20-94% by weight of copper oxide (CuO), preferably 40-92% by weight of CuO, in particular 60-90% by weight of CuO, and   b) 0.005-5% by weight, preferably 0.01-3% by weight, in particular 0.05-2% by weight, palladium and/or a palladium compound (calculated as metallic palladium) and   c) 2-79.995% by weight, preferably 5-59.99% by weight, in particular 8-39.95% by weight, of an oxidic support selected from the group consisting of the oxides of Al, Si, Zn, La, Ce, the elements of groups IIIA to VIIIA and of groups IA and IIA of the Periodic Table of the Elements.

The present invention relates to a process for preparing unsubstitutedor alkyl-substituted y-butyrolactone and tetrahydrofuran by catalytichydrogenation in the gas phase of substrates selected from the groupconsisting of maleic acid and succinic acid and derivatives of theseacids. For the purposes of the present invention, these derivatives areesters and anhydrides which, like the acids, may bear one or more alkylsubstituents. The catalyst used is an all-active catalyst (i.e. acatalyst having the same composition throughout, as distinct from acoated catalyst) comprising copper oxide, palladium and/or a palladiumcompound and at least one further metal oxide.

The preparation of v-butyrolactone (GBL) and tetrahydrofuran (THF) bygas-phase hydrogenation of maleic anhydride (MA) is a reaction which hasbeen known for many years. Numerous catalyst systems for carrying outthis catalytic reaction are described in the literature. These aremostly Cr-containing. Depending on the composition of the catalyst andthe reaction parameters chosen, different product distributions areachieved using such catalysts.

Apart from MA, further possible starting materials for preparing GBL andTHF are maleic acid itself, succinic acid and its anhydride and theesters of these acids. If alkyl-substituted GBL and THF are to beprepared, it is possible to use the correspondingly alkyl-substitutedspecies derived from the abovementioned acids, esters and anhydrides.

U.S. Pat. No. 3,065,243 discloses a process in which copper chromiteserves as catalyst. According to the description and examples,appreciable amounts of succinic anhydride (SA) are formed in thisprocess and have to be circulated. As is known, this frequently resultsin process engineering problems caused by crystallization of the SA orsuccinic acid formed therefrom with subsequent blockage of pipes.

Further copper chromite catalysts for the hydrogenation of MA aredisclosed, for example, in U.S. Pat. No. 3,580,930, U.S. Pat. No.4,006,165, EP-A 638 565 and WO 99138856. According to these disclosures,high yields of GBL can be achieved using the catalysts described there.THF is in each case formed only in traces. However, it is often the casethat relatively large amounts of THF are desired for a number ofreasons.

A process which allows this is disclosed in U.S. Pat. No. 5,072,009. Thecatalysts used according to this patent have the formula CulZnbAlcMdOx,where M is at least one element selected from the group consisting ofgroups IIA and IIIA, VA, VIII, Ag, Au, the groups IITB to VIIB and thelanthanides and actinides of the Periodic Table of the Elements, b isfrom 0.001 to 500, c is from 0.001 to 500 and d is from 0 to <200 and xcorresponds to the number of oxygen atoms necessary to meet the valencecriteria.

Although it is stated that no chromium needs to be present in thecatalysts disclosed in this patent, chromium-containing catalysts aredescribed in all examples. According to these examples, a maximum THFyield of 96% is obtained, and the hydrogenation is carried out atpressures of from 20 to 40 bar.

A two-stage catalyst system for the hydrogenation of MA is described inU.S. Pat. No. 5,149,836. The catalyst for the first stage is free ofchromium while the catalyst for the second stage is based on Cu—Zn—Croxides.

An in-principle disadvantage of all the above-described catalyst systemsis the presence of chromium oxide, whose use should be avoided becauseof its acute toxicity. Such Cr-free catalyst systems for preparing GBLby hydrogenation of MA have also been described in the prior art.Examples of such catalyst systems may be found in WO 99/35139 (Cu—Znoxide), WO 95/22539 (Cu—Zn—Zr) and U.S. Pat. No. 5,122,495 (Cu—Zn—Aloxide). All these catalyst systems make it possible to obtain highyields of GBL, up to 98%, but THF is formed only in traces) if at all.Although the formation of TEF can, as is known, be promoted by anincrease in the reaction temperature or relatively long residence timesin the reactor, the proportion of undesirable by-products, for examplebutanol, butane, ethanol or ethane, increases at the same time.

A catalyst for the gas-phase hydrogenation of MA to GEL which is made upexclusively of Cu and Al oxides is disclosed in WO 97/24346. This, too,suffers from the same disadvantages as the processes disclosed in thedocuments described in the previous paragraph, namely only minorformation of THE which can be increased only by means of extremereaction conditions.

The use of a catalyst having in principle the same composition as thatdescribed in WO 97/24346, namely based on Cu—Al oxides, is alsodisclosed in JP 2 233 631. The aim of this invention is to carry out thehydrogenation of MA in such a way that THF and 1,4-butanediol are formedas main products together with only small amounts, if any, of GBIS. Thisis achieved by the use of the catalysts based on mixed Cu-Al oxides andby adherence to particular reaction conditions. Typical mixturesobtained using this process comprise from about 15 to 20 mol% of1,4-butanediol and from 60 to 80 mol % of THF, with the amount of THFeven being able to be increased to above 99 mol% according to oneexample.

This is achieved by using a large excess of GBL as solvent. In contrast,if no solvent is employed, the yields drop considerably to values in theregion of 75%.

U.S. Pat. No. 4,105,674 discloses a process for the hydrogenation ofmaleic acid, succinic acid or their anhydrides over supported orunsupported Cu-Pd or Cu-Pt catalysts. The aim of that invention is toproduce GBL in high yields and with formation of very small amounts ofby-products such as THF and butanol. For this purpose, a nonacidicmaterial in the examples always magnesium silicate, is preferably usedas support. The catalysts according to that invention achievesselectivities to GBL of over 90%; the selectivity to THF is generallyreported as 2-10%.

All the types of catalyst described in the abovementioned documents havethe disadvantage that they produce a large amount of undesirableby-product or satisfactory activities can be achieved only for thepreparation of GEL. In addition, Cr is frequently present in thecatalyst.

It is an object of the present invention to provide a catalyst for thegas-phase hydrogenation of maleic acid and/or succinic acid and/or theirabove-mentioned derivatives which gives high selectivities tosubstituted or unsubstituted THF. This catalyst should, underappropriate reaction conditions, make it possible to obtain high yieldsof THF with at the same time formation of only small amounts ofundesirable by-product. The catalyst should be free of Cr.

We have found that this-object is achieved by a catalyst for thehydrogenation of C₄-dicarboxylic acids and/or derivatives thereof in thegas phase, which catalyst comprises

-   -   a) 20-94% by weight of copper oxide (Cuo), preferably 40-92% by        weight of CuO, in particular 60-90% by weight of CUO, and    -   b) 0.005-5% by weight, preferably 0.01-3% by weight, in        particular 0.05-2% by weight, palladium and/or a palladium        compound (calculated as metallic palladium) and    -   c) 2-79.995% by weight, preferably 5-59.99% by weight, in        particular 8-39.95% by weight, of an oxidic support selected        from the group consisting of the oxides of Al, Si, Zn, La, Ce,        the elements of groups IIIA to VIIIA and of groups IA and hIA of        the Periodic Table of the Elements.

The catalysts of the present invention allow the hydrogenation ofC₄-dicarboxylic acids and/or derivatives thereof in the gas phase to becarried out so that an unsubstituted or alkyl-substitutedtetrahydrofuran is formed as main product in yields of significantlyabove 90%. It has surprisingly been found that the addition of palladiumas active metal has a significant influence on the selectivity to THF.

For the purposes of the present invention, the groups of the PeriodicTable of the Elements are designated according to the old IUPACnomenclature.

For the purposes of the present patent application, the termC₄-dicarboxylic acids and derivatives thereof refers to maleic acid andsuccinic acid, which may each bear one or more C₁-C₆-alkyl substituents,and the anhydrides and esters of these unsubstituted oralkyl-substituted acids. An example of such an acid is citraconic acid.Preference is given to using MA. The THF produced can, depending on thestarting material used, also bear one or more alkyl substituents.

The catalysts of the present invention comprise copper oxide which isknown per se, palladium and/or a palladium compound, preferablypalladium oxide and/or palladium nitrate, and an oxidic support havingacid centers. It is preferred that no Cr is present in the catalyst. Thecatalysts can be used as shaped bodies, for example as rod extrudates,ribbed extrudates, other extrudate sharpest pellets, rings, spheres andgranules.

The support material can be made up of one or more of the oxides ofelements from the group consisting of Al, Si, Zn, La, Ce, the elementsof groups IIIA to VIIIA and of groups IA and IIA. The support preferablyhas an appropriate number of acid centers. Preference is given to usingan oxide of elements selected from the group consisting of Al, Si, Ti,Zn, Zr and/or Ce. Al is particularly useful. The support is used in anamount of <80% by weight, based on the total catalyst. The amount ofcopper oxide is >20% by weight and the amount of palladium and/orpalladium compound is <5% by weight.

The catalyst of the present invention preferably consists exclusively ofcopper oxide, palladium and/or a palladium compound and aluminum oxide,apart from the usual impurities known to those skilled in the art.

The catalysts of the present invention can further comprise auxiliariesin an amount of from 0 to 10% by weight. For the purposes of the presentinvention, auxiliaries are organic and inorganic materials whichcontribute to improved processing during catalyst production and/or toan increase in the mechanical strength of the shaped catalyst bodies.Such auxiliaries are known to those skilled in the art and includegraphite, stearic acid, silica gel and copper powder.

The catalysts of the present invention are produced by methods known perse to those skilled in the art, for example by coprecipitation of all orat least two components, precipitation of the individual components withsubsequent intimate mixing, for example by kneading or processing in apan mill, impregnation of the oxidic support with the other componentsa) and b) in one or more steps. Furthermore, the catalysts of thepresent invention can be obtained by shaping a heterogeneous mixture ofthe components a), b) and c).

Preference is given to processes in which the copper oxide is obtainedin finely divided form intimately mixed with the support materialparticularly preferably precipitation reactions. The catalyst of thepresent invention can be produced, for example, by precipitation of theappropriate metal carbonates and/or hydroxides from aqueous solution,washing, drying and calcination. The metal carbonates or hydroxides areobtainable, for example, by dissolving the corresponding metal salts inwater and adding a precipitant, preferably sodium carbonate solution.Metal salts used are, for example, nitrates, sulfates, chlorides, 35acetates or oxalates. After precipitation, the precipitate obtained isfiltered off, washed, dried and, if desired, calcined. Palladium and/orthe palladium compound can be precipitated simultaneously with the othercomponents or can be added to the precipitation product at any stage inprocessing.

An active composition comprising component a) or components a) and b)which has been produced in this way can be applied to the support c) ina customary manner, for example by pan milling or kneading, ifappropriate in the presence of a binder, adhesive or peptizing agent. Itis also possible to use other known methods for mixing the supportmaterial c) with an active composition comprising the components a) andb). For example, the support material in the form of powder or shapedbodies can be intimately mixed, coated or impregnated with precursorsubstances of the active composition, for example the abovementionednitrates, sulfates, chlorides, acetates or oxalates or hydroxides of therespective metals or corresponding solutions. This pretreated support isthen subjected to heat treatment to produce the active composition.

However, the catalyst is particularly preferably produced bycoprecipitation of the components a) and c) and subsequent impregnationof the corresponding material obtained after drying, or of the shapedbody obtained after pressing this material, with an aqueous solution ofa soluble palladium compound, in particular a palladium salt solution,for example a solution of the nitrate, acetate, acetylacetonatetpropionate, tetraarninepalladium acetate or tetraaminepalladium nitrate,preferably palladium nitrate. The materials or shaped bodies impregnatedwith the palladium salt solution are subsequently dried, preferably atfrom 50 to 150° C., and, if desired calcined at from 150 to 800° C.Impregnation of a shaped body is preferred.

The catalyst is generally subjected to activation, in general atreatment with hydrogen, before use in the reaction. In this way, theactive catalyst species is produced. This is achieved by partialreduction of the oxides present in the catalyst mixture to the elementalmetal which is active in the catalytic reaction carried out according tothe present invention.

The catalyst of the present invention has a satisfactory operating life.Should, the activity and/or selectivity of the catalyst neverthelessdrop during the operating time, it can be regenerated by means ofmeasures known to those skilled in the art. These include reductivetreatment of the catalyst in a stream of hydrogen at elevatedtemperature. If appropriate, the reductive treatment can be preceded byan oxidative treatment. Here, a gas mixture comprising molecular oxygen,for example air, is passed through the catalyst bed at elevatedtemperature. It is also possible to wash the catalyst with a suitablesolvent, for example methanol THF or GBL, and subsequently to dry it bymeans of a gas stream.

The reaction can be carried out in reactors in which the catalyst isarranged as a fixed bed. Particular preference is given toshell-and-tube reactors which enable the heat liberated in the reactionto be removed in an advantageous manner. MA is vaporized and passedthrough the reactor together with a hydrogen-containing stream of gas.Preference is given to a mixture having a high hydrogen content. In somecases, the addition of other gaseous components such as steam,hydrocarbons, for example methaneo ethane or n-butane, or carbonmonoxide has a favorable effect on the selectivity, activity orlong-term stability.

The concentration of the MA is from 0.1 to 5% by volume, preferably from0.2 to 2% by volume. In the case of significantly higher MAconcentrations, MA condenses out in the reactor and coats the catalystwith a liquid film. Significantly lower concentrations than thoseindicated above are possible in principle, but these would reduce thespace-time yield and make the process unnecessarily expensive. Thereaction temperature is set to a value in the range from 150 to 400° C.,preferably from 200 to 300° C. Higher temperatures promote the formationof by-products, while lower temperatures lead to an unnecessary drop inthe activity of the catalyst. The pressure is set to a value in therange from 0.5 to 50 bar, preferably from 1 to 20 bar. The GHSV (gashourly space velocity=volume flow of reaction gas at STP divided by thevolume of catalyst bed) is set so that complete conversion of MA isachieved. This makes the work-up of the product mixture easier and savesrecirculation of unreacted MA. The GHSV is from 10 to 50 000 h⁻¹,preferably from 100 to 10 000 h⁻. The product mixture can be separatedby methods known to those skilled in the art. Preference is given tocirculating at least part of the unreacted hydrogen and thus reusing itin the hydrogenation.

It has been found that the formation of the desired end products can becontrolled by variation of the reaction parameters. These are, inparticular pressure, temperature and GHSV. Thus, an increased, sometimeseven exclusive, formation of THF is generally observed at high pressuresand temperatures and low GHSV values. In contrast, low pressures andtemperatures and high GHSv values lead to increased formation of GEL.

The invention is illustrated by the following examples.

COMPARATIVE EXAMPLE C1 Production of CuO/Al₂O₃ Catalyst Pellets

3 l of water are placed in a heatable precipitation vessel fitted with astirrer and are heated to 80° C. A metal salt solution consisting of1754 g of Cu(NO₃)₂*2.5H₂O and 2944 g of Al(NO₃)₃*9H₂O in 4000 ml ofwater are metered simultaneously with a 20% strength by weight solutionof sodium carbonate into this precipitation vessel over a period of onehour while stirring. The amount of sodium carbonate metered in isselected so that a pH of 6 is established in the precipitation vessel.After all the metal salt solution has been added, further sodiumcarbonate solution is metered in until the pH in the precipitationvessel has reached 8, and the mixture is stirred at this pH for another15 minutes. The total consumption of sodium carbonate solution is 11 kg.The suspension formed is filtered and the solid is washed with wateruntil the washings no longer contain nitrates (<25 ppm). The filter cakeis firstly dried at 120° C. and subsequently calcined at 600° C.

600 g of this material are intimately mixed with 18 g of graphite andtabletted to produce pellets having a diameter of 3 mm and a height of 3mm.

EXAMPLE 2 Production of a CuO/Pdb/Al₂O₃ Catalyst According to thePresent Invention

550 g of the pellets from Example 1, which had a water uptake capacityof 0.33 cm³/g, were uniformly sprayed in an impregnation drum with asolution of 2.76 g of Pd as palladium nitrate in 172 ml of water, driedat 100° C. and finally calcined at 350° C. for 2 hours.

COMPARATIVE EXAMPLE C3 Production of a CuO/PtO/Al₂O₃ Catalyst forComparison:

550 g of the pellets from Example 1, which had a water uptake capacityof 0.33 cm³/g, were uniformly sprayed in an impregnation drum with asolution of 2.76 g of Pt as platinum nitrate in 172 ml of water* driedat 100° C. and finally calcined at 350° C. for 2 hours.

COMPARATIVE EXAMPLE C4, EXAMPLE 5, COMPARATIVE EXAMPLE C6 Hydrogenationof Maleic Anhydride

100 ml of the catalyst pellets from Comparative Example C1 or Example 2were in each case mixed with 100 ml of glass rings of the same size andplaced in a tube reactor having an internal diameter of 27 mm. Thetemperature of the reactor was regulated by means of oil flowing aroundit, and the reaction gas was passed through the reactor from the topdownwards. MA was pumped as a melt into a vaporizer operated at 200° C.where it was vaporized in a stream of water The MA/hydrogen mixture,which had an MA concentration of 1.2% by volume, was then passed throughthe reactor and preheated above the catalyst bed. Complete conversion ofMA was obtained in all examples.

Before the MA/hydrogen mixture was fed into the reactor, the catalystwas subjected to a pretreatment with hydrogen. For this purpose, 200standard 1/h of nitrogen were firstly passed through the reactor underatmospheric pressure and the reactor was simultaneously heated to atemperature in the catalyst bed of 180° C. over a period of one hour.The nitrogen flow was then increased to 950 standard 1/h and anadditional 50 standard 1/h of hydrogen was fed in. A slight temperatureincrease in the catalyst bed to about 250° C. at the hot spot wasobserved. The hot spot migrates through the reactor from the reactorinlet to the end of the reactor. After the temperature had dropped to190° C. throughout the catalyst bed, the nitrogen flow was reduced to900 standard 1/h and the water flow was increased to 100 standard 1/h.The nitrogen flow was gradually switched off and the hydrogen flow wasgradually increased to 250 standard 1/h.

To compare the activity of the catalysts, the GHSV was increased from2500 to 6000 h⁻¹.

Space- time S_(THF) S_(others) yield Ex. Cat. T GHSV S_(BA) S_(GBL) [mol[mol [g_(THF)/ No. No. [° C.] [l/h] [mol %] [mol %] %] %] hl_(cat.)] C4C1 250 2500 0 0 93 7 89 250 3000 <1 13 83 7 95 250 6000 30 62 7 1 16 5 2250 2500 0 0 89 11 85 250 3000 0 0 91 9 104 250 6000 0 0 93 7 212 C6 C3250 2500 0 2 76 22 73 250 3000 10 71 15 4 17 250 6000 32 59 7 2 16S_(xxx) = selectivity to the respective product

As can be seen from the table, in the case of the Pd-free catalyst ofComparative Example 1, the maximum space-time yield of tetrahydrofuranis achieved at a GHSV of 3000 h⁻¹. The THF selectivity of 93% isachieved by the catalyst at a space-time yield of 89 g_(THF)/h1_(catalyst).

In comparison thereto, the catalyst according to the invention fromExample 2 achieves an increase in the space-time yield to 212 g_(THF)/h*1_(catalyst) at the same tetrahydrofuran selectivity of 93%.The platinum-doped catalyst from Comparative Example 3, on the otherhand displays a poorer activity and selectivity to tetrahydrofurancompared to the undoped system.

1-4. (canceled)
 5. A process for the hydrogenation of C₄-dicarboxylicacids and/or derivatives thereof, in the gas phase in the presence of acatalyst, wherein said catalyst comprises 40-92% by weight of copperoxide 0.005-5% by) weight of palladium and/or of a material having acidsites, which support material is selected from the group consisting of:oxides of Al, Si, Zn, La, Ce, elements of groups IIIA to VIIIA andelements of groups IA and IIA.
 6. A process as claimed in claim 5,wherein the GHSV is from 10 to 50 000 h⁻¹.
 7. A process as claimed inclaim, wherein the reaction temperature is from 150 to 400° C., and thepressure during the reaction is from 0.5 to 50 bar.
 8. A process asclaimed in claim 5, wherein the concentration of C₄-dicarboxylic acid orthe derivative thereof is from 0.5 to 5% by volume.
 9. (canceled) 10.The process for the hydrogenation of C₄-dicarboxylic acids and/orderivatives thereof as claimed in claim 5 for the hydrogenation ofmaleic anhydride.
 11. The process of claim 5, wherein the catalystcomprises palladium oxide and/or palladium nitrate.
 12. The process ofclaim 5, wherein the support material is selected from the groupconsisting of aluminum oxide, silicon oxide, titanium oxide, zinc oxide,zirconium oxide and cerium oxide.
 13. The process of claim 5, whereinthe catalyst consists of copper oxide, palladium and/or a palladiumcompound and aluminum oxide.
 14. The process of claim 5, wherein theGHSV is from 100 to 10 000 h⁻¹.
 15. The process of claim 7, wherein thereaction temperature is from 200 to 300° C.
 16. The process of claim 7,wherein the pressure during the reaction is from 1 to 20 bar.
 17. Theprocess of claim 8, wherein the concentration of C₄-di-carboxylic acidor the derivative thereof is from 0.2 to 2% by volume.